The invention relates to a particulate material to be used as filling material in the heat-sensitive layer of a heat-sensitive recording material, particularly a thermoreactive paper.
A heat-sensitive recording material, for example a thermoreactive paper, usually contains a carrier and a heat-sensitive recording layer produced on the carrier. This recording layer is essentially composed of an electron-donating, colorless or only slightly colored dye precursor (leuco dye) and an electron-accepting dye developer, in most cases a phenolic compound. When heated with a thermal print head, the dye precursor instantly reacts with the dye developer, thus creating a recorded image or a dye-developer complex.
These heat-sensitive recording materials are widely used in facsimile machines and printers and as labeling paper. Without doubt, the main area of application is the operation of facsimile machines, where the continuously increasing transmission speed causes a demand for ever more sensitive recording materials. This demand is satisfied by the addition of sensitizing agents that accelerate the color reaction as well as by fine-grained pigments which are used to prevent the adhesion of the molten dye-developer complex to the thermal print head.
These fine-grained filling pigments are characterized by a high oil absorption of generally &gt;100 ml 100 g, a high particle fineness with a mean secondary particle size of generally at least 70 percent by weight &lt;4 .mu.m (measured according to the centrifugal sedimentation method). Also, they are characterized by a surface activity as small as possible, that is, a specific surface generally of &lt;100 m.sup.2 /g (determined according to the BET-method). The latter is significant for avoiding background dyeing on the thermoreactive papers, Which can occur as a result of an undesired color reaction of the dye precursor with an overly reactive dye developer pigment.
Well-known paper pigments, such as calcium carbonate, kaolin, talc, aluminum hydroxide, titanium dioxide or synthetic organic pigments are used as white pigments. However, none of these pigments or pigment mixtures fulfills all of the criteria mentioned above. For example, they lack sufficient oil absorption.
From German DE-A-3, 942, 692 (U.S. Pat. No. 4,985,394), a heat-sensitive recording material is known which contains a carrier sheet underneath a heat-sensitive dyeing layer with a leuco dye and a dye developer, and this heat-sensitive dyeing layer is covered with a protective layer. The protective layer contains at least one inorganic pigment from the group of silicic acid and calcium carbonate, each with an average particle diameter of 0.1 .mu.m or less and a water-soluble binding agent. Therefore, the pigment is not located in the heat-sensitive dyeing layer. This design has the disadvantage that the protective layer constitutes a barrier for heat transmission, impairing efficient action of the thermal print head on the heat-sensitive layer of the recording material. Furthermore, silicic acid and calcium carbonate occur as a mechanical mixture.
From German DE-A-3,911,199, heat-sensitive recording materials are known which contain an intermediate layer of colloidal silicon dioxide and/or aluminum oxide between a carrier and the heat-sensitive color-producing layer. The silicon dioxide can also occur as a mechanical mixture with a white pigment, such as calcium carbonate, that is, these two components are not bound to one ,another. Moreover, they are not localized in the heat-sensitive color-producing layer, but rather in the intermediate layer functioning as a reflective layer.
From German DE-A-3,019,591 (U.S. Pat. No. 4,414,259), a heat-sensitive recording material is known that contains an oil-absorbing pigment, such as finely dispersed silicon dioxide, and an inorganic pigment, such as calcium carbonate, in its dye-developing layer. These two substances, however, occur only as a mechanical mixture and are not bound together.
From German DE-0-2,800,485 (U.S. Pat. No. 4,168,845 and 4,311,758), a similar heat-sensitive recording material is known that also contains the finely dispersed silicon dioxide and the white pigment only in the form of a mechanical mixture.
From European Patent B1-O-114,749, an amorphous silicate is known which is suitable filling material for thermoreactive papers. Its production is performed by converting alkali silicate with mineral acid in a concentrated alkali salt solution, in which process initially fine-grained amorphous silicic acid is produced directly by avoiding the sol state. With a metal hydroxide, the silicic acid is converted to an amorphous silicate exhibiting the required low surface activity with specific surfaces of &lt;100 m.sup.2 /g, as well as an oil absorption of 100 to 200 ml/100 g. The secondary particle size of 90 percent by weight is &lt;4 .mu.m (measured according to the centrifugal precipitation method), the bulk density is reported as 0.14 to 0.30 g/cm.sup.3. The disadvantage of this process is that large amounts of concentrated salt solutions have to be used which can lead to an enormous disposal problem and thus can cause environmental problems.
From European Patent B1-0 135,976, a particulate material to be used as filling material in the heat-sensitive layer of heat-sensitive recording paper is known. This material contains fine-grained amorphous silicic acid with a specific BET-surface of 10 to 100 m.sup.2 /g and a bulk density of 0.14 to 0.3 g/cm.sup.3 ; the fine-grained amorphous silicic acid has a mean particle size distribution of at least 90 percent by weight &lt;4 .mu.m. The production of this particulate material is carried out by converting an alkali silicate with an acid in a concentrated alkali salt solution by which process fine particles of a silica gel are precipitated while the sol state is avoided. Again, the disadvantage of this process is that large amounts of concentrated salt solutions have to be used.
Surprisingly, it was found that otherwise well-known filling materials for paper which exhibit an oil absorption too low for them to be used in heat-sensitive recording materials, can be combined with precipitated silicic acid to form filling materials which are very well suited as thermoreactive pigments with respect to oil absorption, particle fineness and surface activity.